As electrophotography, a method of forming an electrostatic charge image on a photosensitive material, developing the electrostatic charge image by a toner which is carried on a developing roller, transferring the toner image developed on the photosensitive material directly to a recording medium, e.g., paper, or via an intermediate transfer substance, and fixing the toner image on the recording medium by a fixing roller, e.g., a heating roller, on the recording medium, e.g., paper, by press-heating is known.
For obtaining high quality images, the toners used in such a fixing unit by heating fixation are required to have excellent fixing characteristics that a recording medium surely separates from fixing members and does not wind round fixing members after heating fixation at the exit of a fixing unit, and toners do not bring about a so-called hot offset, i.e., the adhesion of melted toners on a heating roller.
In fixation using a heating roller, as the factors which control the fixing property and offset resistance of a toner, it is well known that the storage modulus G′ and the loss modulus G″ in dynamic viscoelastic characteristics of a toner have influence. Storage modulus G′ and loss modulus G″ are viscoelastic characteristics of a substance having general viscoelasticity defined by complex elastic modulus in vibration experiment, and the real number part of complex elastic modulus is called storage modulus G′ and the imaginary number part is called loss modulus G″, specifically, storage modulus is made an index showing the degree of the elasticity of a toner and loss modulus is made an index showing the degree of viscosity.
In general, toners containing a binder resin comprising a polymer show a stress relaxation behavior that generated stress exponentially attenuates when constant strain is given. Therefore, for the purpose of the improvement of fixing property of toner images, such as offset of toners to a fixing roller, and the fixing strength of toners on a recording medium, e.g., paper, and the improvement of dispersibility of colorants, the relaxation modulus and relaxation time of toners which can be obtained by measuring the dynamic viscoelasticity have been conventionally used for confirming the characteristics of toners quantitatively.
Since such a stress relaxation behavior of a toner is greatly influenced by the viscoelasticity of a binder resin and the structure, size and quantity of a release agent dispersed in the resin, the molten condition of a toner can be expressed with a stress relaxation behavior, i.e., relaxation modulus G (t) and relaxation time. It is proposed, therefore, to improve the stability of a toner image, the dispersibility of a release agent, and OHP transparency by the expressing molten condition of a toner at fixing time with relaxation modulus G of viscoelastic is characteristics and relaxation time.
That is, in the toner in this proposal, it is proposed to improve the separability of a fixed sheet, the adhesion property of a fixed image, the bending resistance of a fixed image, the dispersibility of the release agent in a toner, the dispersibility of the colorant in a toner, and fixing characteristics, such as OHP transparency, by prescribing the initial relaxation modulus G (t=0.01) in relaxation time of 0.01 sec obtained by the measurement of the dynamic viscoelastic characteristics of the binder resin of a toner within a prescribed range, and, considering the heating time Dt in fixation, by prescribing the ratio G (r) of the initial relaxation modulus G (t=0.01) to the initial relaxation modulus G (t=0.1) in relaxation time of 0.1 sec, [G (t=0.01)/G (t=0.1)], within a prescribed range. (Refer to, e.g., patent literature 1).
On the other hand, the configurations of the fixing nip parts of fixing members used in fixing units by heating fixation are various.
For instance, fixing units having a fixing nip part of a heating roller and a pressing roller of fixing members taking the configuration of protruding toward the pressing roller side are known (refer to, e.g., patent literature 2).
For instance, fixing units having a fixing nip part of a heating roller and a pressing roller of fixing members and a pressing belt taking the configuration of protruding toward the side of the main heating member are known (refer to, e.g., patent literature 3).
For instance, fixing units having a fixing nip part of a fixing film for heating and a pressing roller taking the configuration of flat surface are known (refer to, e.g., patent literature 4).
[Patent Literature 1]
Japanese Patent No. 3241003 (paragraphs [0020], [0030], [0031] and [0051]).
[Patent Literature 2]
JP-A-2002-268430 (paragraphs [0014], [0015] and FIG. 3) (the term “JP-A” as used herein refers to an “unexamined published Japanese patent application).
[Patent Literature 3]
Japanese Patent 3322095 (paragraphs [0022] to [0026] and FIGS. 1 and 3).
[Patent Literature 4]
JP-A-9-22772 (paragraphs [0004] to [0008], FIGS. 7 and 8)
Various different nip configurations have a great influence on the direction of discharge of a recording medium when discharged from a fixing unit.
For example, in the fixing unit disclosed in patent literature 2, since the fixing nip part of the heating roller and the pressing roller takes the configuration of protruding toward the pressing roller side, the direction of discharge of a recording medium tends to be the direction along the heating roller. Due to this tendency, the recording medium is liable to wind round the heating roller after heating fixation.
However, in the toner disclosed in patent literature 1, it is disclosed to improve fixing characteristics, such as fixing strength, of a toner image by expressing the molten state of a toner by the ratio G (r) of the relaxation modulus G (t), where heating time Dt at fixing time is considered, and the relaxation modulus G (t) after the elapse of certain relaxation time. However, the nip configuration of the fixing unit is not supposed at all in the same patent. Therefore, there remains a problem that it is difficult to effectively prevent a recording medium from winding round the heating roller.
Further, when a toner is made soft for the purpose of improving fixing strength and the like, not only the winding of a recording medium round a heating roller is liable to occur, but hot offset is also liable to occur by the reluctance of separation of softened toner.
Further, for example, in the fixing unit disclosed is in patent literature 3, since the fixing nip part of the heating roller and the pressing belt takes the configuration of protruding toward the heating roller side, the direction of discharge of a recording medium tends to be the direction forcedly released from the heating roller. Due to this tendency, the toner on the recording medium is forcedly peeled off the medium, and hot offset after heating fixation is liable to occur by the reluctance of separation of the toner adhered on the heating roller side and that adhered to the recording medium side.
However, in the toner disclosed in patent literature 3, it is disclosed to improve fixing characteristics, such as fixing strength, of a toner image by expressing the molten state of a toner by the ratio G (r) of the relaxation modulus C (t), where heating time Dt at fixing time is considered, and the relaxation modulus G (t) after the elapse of certain relaxation time. However, the nip configuration of the fixing unit as disclosed in patent literature 3 is not supposed at all in the same patent. Therefore, there remains a problem that it is difficult to effectively prevent hot offset after heating fixation by the reluctance of separation of the toner.
In a fixing unit as disclosed in patent literature 3, where a fixing nip part takes the configuration of protruding toward the heating roller side, a recording medium tends to peel off the heating roller, nonetheless it is difficult to surely prevent the winding of the recording material around the heating roller.
Further, for example, in the fixing unit disclosed in patent literature 4, since the fixing nip part of the fixing film for heating and the pressing roller takes the configuration of flat surface, a recording medium tends to be discharged along the direction of the flat surface of the nip configuration. Due to this tendency, the toner on the recording medium is half forcedly peeled off the fixing film, and hot offset after heating fixation is liable to occur by the reluctance of separation of the toner adhered on the fixing film side and the toner adhered to the recording medium side.
However, in the toner disclosed in patent literature 1, it is disclosed to improve fixing characteristics, such as fixing strength, of a toner image by expressing the molten state of a toner by the ratio G (r) of the relaxation modulus G (t), where heating time Dt at fixing time is considered, and the relaxation modulus G (t) after the elapse of certain relaxation time. However, the nip configuration of the fixing unit as disclosed in patent literature 4 is not supposed at all in the same patent. Therefore, there remains a problem that it is difficult to effectively prevent hot offset after heating fixation by the reluctance of separation of the toner.
In a fixing unit as disclosed in patent literature 4, where a fixing nip part takes the configuration of flat surface, a recording medium tends to peel off the fixing film, nonetheless it is difficult to surely prevent the winding of the recording material around the fixing film.